of the Cuzco Valley, Peru. 



27 



walls which, truncate the slopes at high angles. This feature 

 is well shown at Angostura Narrows (figs. 14 and 20) and 

 also at Huambutio. The relations of these three slopes, devel- 

 oped on rock of essentially uniform composition and structure, 

 require for their interpretation the assumption of three erosion 









Fig. 16. 





RUMI 



-CCOLCA 



PIQUI-LLACTA RIO r URUBAMBA 



12., OOO - 









- IE. ooo 



II ooo - 









- II, ooo 













s 



w. 



O 



N 

 Z MILES 



=> ' 



E.. 



Fig. 16. Cross profile of Urubamba Valley and lower end of Cuzco 

 Valley, drawn through Puquillacta. 



cycles — the regional base-leveling of the Inca peneplain, the 

 recent cutting of the present stream valleys, and an intermediate 

 cycle during which the mature slopes were formed. 



The lower margins of mature slopes bordering the Huatanay 

 are in most places buried by accumulations of gravel brought 



PIQUI-LLAGTA 



Fig. 17. 



HUAMBUTIO 



S.E 



Fig. 17. Cross profile, Huatanay Valley at Huambutio. 



down during Pleistocene time by swift streams from the high- 

 lands. West of Cuzco and north of San Geronimo, where 

 bedrock is exposed by deep-cut trenches traversing alluvial fans, 

 the mature slopes continue beneath the materials of Pleistocene 

 age. 



